High altitude and long endurance aircraft such as certain unmanned air vehicles (UAV's) may require wings having a long span and a high aspect ratio. Such UAV's may be configured to operate in a loitering capacity at relatively high altitudes (e.g., 50,000 to 80,000 feet) for prolonged periods of time to provide coverage of a specific geographic area. The coverage may include intelligence gathering, surveillance and reconnaissance operations wherein information may be gathered by the UAV and transmitted to ground units or other air units.
Although UAV's may be ground-launched from a runway in the conventional manner for aircraft, in some applications it is desirable to deploy the UAV as a payload of another vehicle. For example, in applications wherein the geographic area where surveillance is desired is far away such as overseas, it may be desirable to transport the UAV to a chosen location such as via an intercontinental ballistic missile (ICBM) or within the payload bay of a larger aircraft such as a long-range bomber or airlifter. Upon reaching the desired location, the UAV may be separated from the ICBM or air-dropped from the larger aircraft.
In order to permit packaging of the UAV into the small confines of an ICBM or into an aircraft for aerial drop, it is necessary to stow the relatively large wings and control surfaces into very small volumes. One preferred option for stowing the wings is to configure the wings to be compactable in the payload bay of the missile or larger aircraft such that the wings can be inflated following deployment of the UAV. Furthermore, for aircraft designed to operate in hostile airspace, it is also desirable to minimize the radar visibility or signature of the loitering UAV to increase its survivability. One method for minimizing radar visibility in aircraft is to use materials that are non-reflective to radar such as certain non-metallic materials.
Fortunately, most materials used in the construction of inflatable wings are non-metallic such that the wings themselves are typically radar transparent. However, while the wing structure may be radar transparent, current mechanisms for actuating the control surfaces of the wings such as ailerons, flaps and leading edge devices require the use of mechanisms and/or materials that may not be radar transparent. Even if such current mechanisms are inherently radar transparent, they may possess certain drawbacks and deficiencies that detract from their overall utility.
For example, one option for minimizing the radar transparency of control surfaces is to eliminate the control surfaces altogether. However, because most aircraft require some type of mechanism for controlling the aircraft flight attitude, it is necessary to relocate the control mechanism to the propulsion system. Unfortunately, arranging the propulsion system to provide the aircraft with directional flight control capability may necessitate the use of independently controllable throttles on at least two separate propulsion units.
Furthermore, providing directional control via the propulsion units requires the use of thrust-vectoring devices integrated within the propulsion units. Although such propulsion systems are available, they are also necessarily complex, costly, bulky and heavy. A further drawback associated with the use of the propulsion system as the source of directional control is that a loss of power in the propulsion system not only results in a loss of propulsive force to the aircraft, but also a loss in directional control of the aircraft.
Another option for actuating the control surfaces of an aircraft is through the use of electromechanical actuators. Such electromechanical actuators may be mounted in the wing and may be used to manipulate the flight control surfaces (e.g., ailerons, flaps, etc.) in order to control flight direction and attitude. Unfortunately, because most electromechanical actuators are constructed of metallic materials, they typically exhibit high radar reflectivity. Furthermore, many electromechanical actuators include electric motors constructed with ferrous materials that also exhibit high radar reflectivity. Even further, power is typically provided to the electric motors through the use of metallic wiring extending through the aircraft and which act as radar antennae when extending through the wings to the electromechanical actuators.
A further option for actuating the control surfaces of an aircraft include the use of piezoelectrics wherein piezoelectric strips are mounted on and/or under the surfaces of the inflatable wings such as near the trailing edge. A positive or negative voltage is applied to the piezoelectric strips to cause the strips to expand or contract and therefore curve upwardly or downwardly. If the strips are mounted on the wing near the trailing edge, the trailing edge is also caused to curve upwardly or downwardly such that the portion of the trailing edge may function as an aileron or flap. Unfortunately, the piezoelectric strips operate via dielectric (i.e., voltage) potential which is the very mechanism by which radar sees a surface and therefore rendering such piezoelectric strips unsuitable for use in aircraft requiring radar transparency.
An even further option for actuating control surfaces includes wing warping techniques such as that which was employed by the Wright Brothers for roll control of their experimental aircraft. Wing warping is facilitated through the use of a system of cables and pulleys for twisting the trailing edges of the wings. Unfortunately, the use of wing warping on certain aircraft may result in certain disadvantages such as aerodynamic drag due to exposed cables. Although modern cables are available in radar transparent materials, the exposed cables impose a significant aerodynamic drag penalty which increases with the increasing speed of the aircraft.
As can be seen, there exists a need in the art for a system and method for actuating a control surface that has low radar visibility or is radar transparent. Furthermore, there exists a need in the art for a system and method for actuating a control surface that is predictable and which does not impose excessive aerodynamic drag penalties on the aircraft. Additionally, there exists a need in the art for a system and method for actuating a control surface that is of simple construction, light weight and low cost.